Field of the Invention
The present invention relates to a light scanning apparatus and an image forming apparatus including the light scanning apparatus.
Description of the Related Art
Hitherto, in an image forming apparatus employing an electrophotographic method, a photosensitive drum having a surface charged to a uniform electric potential is scanned with a light beam which is emitted from a light scanning apparatus based on image information, to thereby form an electrostatic latent image. The formed electrostatic latent image is developed by developer (toner) into a visible image, and the visible toner image is transferred onto a sheet. After that, the unfixed toner image is fixed on the sheet by a fixing unit, and the sheet is delivered. The light scanning apparatus configured to perform scanning with a light beam includes an optical system having a deflection device, which includes a rotary polygon mirror configured to deflect the light beam emitted from a semiconductor laser serving as a light emitting source, an optical lens (fθ lens), a reflecting mirror, and other components. In recent years, there has been an increasing demand for high-speed recording in the image forming apparatus, and hence higher scanning speed in the light scanning apparatus, that is, higher rotation speed of the rotary polygon mirror of the deflection device has been pursued continuously. When the rotary polygon mirror is rotated at high speed, a positive pressure region and a negative pressure region are generated on a mirror surface of the rotary polygon mirror, thereby causing adhesion of dirt, such as fine dust or mist in air, on the negative pressure region of the mirror surface. Such adhesion of dirt on the rotary polygon mirror reduces reflectivity at a portion with the adhesion of dirt. Therefore, the light intensity of a light beam, which is deflected by the rotary polygon mirror and output from the light scanning apparatus, is reduced. There has been a problem in that the reduced light intensity may cause failure in writing to a photosensitive drum as well as image degradation on the sheet onto which an image formed on the photosensitive drum is transferred.
To address this problem, a related-art light scanning apparatus secures sealability of the light scanning apparatus with the following configuration. Specifically, an opening formed on top of a housing having optical components mounted therein (hereinafter referred to as “optical box”) is covered with a cover component (hereinafter referred to as “upper cover”) configured to cover the opening, and a sealing part formed of a soft sealing member such as a foam member is sandwiched at a portion where the optical box and the upper cover are brought into abutment against each other. The upper cover and the optical box are snap-fitted or fastened with screws. With this, the sealing member of the sealing part is pressed, thereby securing the sealability of the light scanning apparatus.
In the case of this configuration, there may occur deformation of the upper cover due to a repulsion force of the pressed sealing member, and fatigue degradation of the sealing member due to continuous pressing. Thus, there is a problem in that the sealability of the light scanning apparatus is degraded along with deformation of the upper cover or fatigue degradation of the sealing member. For the purpose of preventing degradation in the degree of sealing of the optical box, there has been proposed a measure to reduce deformation of the upper cover and fatigue degradation of the sealing member to a maximum extent.
For example, in Japanese Patent Application Laid-Open No. 2014-12368, there is proposed an optical box including a plurality of fixing seats for allowing an upper cover to be fastened thereon with screws. In this optical box, the fixing seats onto which the upper cover is screwed include high fixing seats and low fixing seats. When use is started, the high fixing seats and the upper cover are fastened with screws. The fixing seats are removable. When the sealing member is degraded by fatigue, the upper cover is re-assembled, and the high fixing seats having been used so far are removed. Then, the upper cover is screwed onto the low fixing seats, thereby being capable of fastening the upper cover even under a state in which the sealing member is degraded by fatigue. Removing the high fixing seats and screwing the upper cover onto the low fixing seats can suppress deformation of the upper cover due to a repulsion force from the fixing seats.
In the method described above, there is a problem in that the dust-proof performance may be degraded due to a gap formed between the sealing member and the upper cover along with deformation of the upper cover which may occur from an initial period of use rather than over time, and due to separation of the sealing member caused by the deformation of the upper cover.